(a) Field of the Invention
The present invention relates to a power saving A/D converter and, more particularly, to an A/D converter suited for use in a portable data communication system such as a personal data assistant (PDA).
(b) Description of the Related Art
An A/D converter is used in a portable data communication system for converting an analog input signal, such as an audio signal or image signal, to a digital output signal for data transmission.
JP-A-3(1991)-185928 proposes an A/D conversion system which repeats A/D conversion in a cyclic operation. FIG. 1 shows the proposed A/D conversion system, which comprises an A/D converter 12 for effecting A/D conversion and a control section 36 having therein an interval counter (not shown) for counting the clock pulses in the interval clock input 41.
Referring additionally to FIG. 2 showing the timing chart for the A/D converter of FIG. 1, after the interval counter in the control section 36 counts up to the setting of number of counts specified by the interval setting signal 19, the interval counter supplies a conversion start signal 38 to the A/D converter 12. The A/D converter 12 effects A/D conversion of the input analog signal 14, sets the output digital signal 20 after the completion of the conversion and supplies an end of conversion (EOC) signal 40 to the control section 36. The control section 36 then supplies a conversion stop signal 39 and activates the interval counter at the same time.
Subsequently, the interval counter again counts the clock pulses in the interval clock input 41 and supplies a conversion start signal 38 after the count up of the specified setting. These steps are repeated for a cyclic A/D conversion.
In a portable data communication system, power saving is increasingly requested for a longer operational period of a battery. However, the conventional A/D converter as described above does not sufficiently meet this request.